Set the OS name in the metrics protobuf file

There are cases where the developer is not building a product so
the metadata information is not being set. For now, set the OS
name so we can track which OS the user is using it.

Fixes: b/163118212
Test: m nothing and checked the blueprint
Change-Id: I9b521eee2d266b50625aa8e916ddaacb4e2feaea